(1) Field of the Invention
The present invention relates to a variable-threshold-type differential signal receiver and more particularly to a differential signal receiver useful in a data transfer system of, for example, a Local Area network (usually referred to as a LAN), in which the threshold voltage for providing logical outputs is made variable.
(2) Description of the Prior Art
A differential-type signal receiver including a differential amplifier is known. The differential-type signal receiver receives differential input signals having a differential voltage and determines whether the differential voltage is high or not with respect to a predetermined threshold voltage so as to provide logical output signals corresponding to the differential voltage of the above-mentioned differential input signals. The differential input signals ae usually transmitted through a transformer and a pair of transmission lines connected to the secondary wiring of the transformer to the differential-type signal receiver. The differential input signals, such as a packet in the LAN, consist of a series of a high-potential signal and a low-potential signal when a signal is transmitted through the transformer. That is, the waveforms of the received input signals have edges. Whether the differential voltage is high or not is determined by detecting the edges of the waveforms of the input signals.
Conventionally, the predetermined threshold voltage of a differential-type signal receiver is usually fixed to be zero volts. This causes a problem in that, when no signal is transmitted through the transformer to the differential-type signal receiver, the input signals are zero volts and have no edge. Therefore, the differential-type signal receiver cannot determine whether the input signals have a high potential or a low potential with respect to a threshold voltage of zero volts. In this case, the output of the differential-type signal receiver may oscillate or may be easily influenced by noises, causing the output signal of the differential-type signal receiver to be indefinite. When a unit receives an indefinite signal from the output of the differential-type signal receiver, the unit may malfunction.
Therefore, it is preferable to make the differential-type signal receiver output a constant "1" or "0" level signal when it receives a zero-volts signal.